1. Field of the Invention
The invention relates to addition-crosslinkable silicone compositions, the vulcanizates of which have low coefficients of friction, to their preparation, and to moldings composed of the crosslinked silicone compositions.
2. Background Art
It is known that addition of low-viscosity silicone oils which have some incompatibility with silicone elastomers can reduce the coefficient of friction of the elastomers. The additives are generally phenyl-containing silicone oils whose viscosity is from 5 to 1000 mPas, which are added to the uncrosslinked addition-crosslinking silicone composition. The phenyl-containing silicone oils have sufficient solubility in the uncrosslinked silicone composition to prevent phase separation. After crosslinking, the phenyl-containing silicone oils exude from the crosslinked silicone rubber, and an oily film thus forms on the surface over the course of time, reducing the coefficient of friction. The disadvantage of these mixtures is, however, that the oily friction reducing film is not formed immediately after crosslinking but only after some hours. Furthermore, the exudation rate is time- and temperature-dependent, and the coefficient of friction therefore depends on the storage conditions. Another problem with these systems is that the oil continues to exude, and constant oil content on the silicone surface is therefore achieved only after some weeks. Furthermore, the low-molecular-weight oil that has migrated to the surface is unbound and can simply be removed by wiping. This method cannot ensure a long lasting low coefficient of friction. Another consequence of the silicone oil migrating to the surface is that when the finished parts are conveyed the silicone oil film causes silicone-oil contamination of the conveyor belts, subsequently requiring complicated cleaning.
EP 649877 indicates that addition of incompatible low-viscosity silicone oils, e.g. silicone oils having dimethylsiloxy and diphenylsiloxy units or arylphosphoric esters, to addition-crosslinking or peroxidically crosslinking compositions can produce a lubricating film on the surface of the crosslinked silicone elastomer. However, this lubricating film is developed only after one day, and the coefficient of friction is therefore reduced only after storage of the crosslinked parts.